Generally, with the object of protecting and reinforcing the bare optical fiber threads, an optical fiber is provided with a resin coating comprising a flexible primary coating layer on the surface of the optical fiber immediately after hot melt spinning of the glass fiber, and a flexible secondary coating layer which covers the surface of the primary coating layer. The coating material used to form these coatings must have the following characteristics:
(1) be a liquid at room temperature and have high processability; PA1 (2) provide good productivity at a high curing rate; PA1 (3) show superior flexibility; PA1 (4) exhibit very little physical change during temperature changes over a wide range; PA1 (5) have superior thermal resistance such as resistance to heat and thermal aging and superior resistance to hydrolysis; PA1 (6) show superior long term reliability and little physical change with passing time; PA1 (7) show superior resistance to chemicals such as acids and alkalis; PA1 (8) exhibit low moisture absorption; and PA1 (9) exhibit superior light resistance including resistance to ultraviolet light. PA1 (B) a polyisocyanate compound; and PA1 (C) at least one (meth)acrylate compound containing a hydroxyl group. PA1 (1) A method wherein the polyol compound (A), the polyisocyanate compound (B), and the (meth)acrylate compound containing a hydroxyl group (C) are all charged and reacted together; PA1 (2) A method wherein the polyol compound (A) and the polyisocyanate compound (B) are reacted together and then reacted with the (meth)acrylate compound containing a hydroxyl group (C); PA1 (3) A method wherein the polyisocyanate compound (B) and the (meth)acrylate compound containing a hydroxyl group (C) are reacted together and then reacted with the polyol compound (A); and PA1 (4) A method wherein the polyisocyanate compound (B) and the (meth)acrylate compound containing a hydroxyl group (C) are reacted together and then reacted with the polyol compound (A), and finally the product is again reacted with the (meth)acrylate compound containing a hydroxyl group (C).
With respect to these requirements, various types of radiation-curable liquid coating materials have been developed, for example, materials containing urethane acrylates using a tetrahydrofuran-ethyleneoxide ring-opening copolymer (Japanese Laid-Open Patent Application No. 86450/1986), a tetrahydrofuranpropyleneoxide ring-opening copolymer (Japanese Laid-Open Patent Application No. 181170/1985), or a tetrahydrofuranalkyltetrahydrofuran copolymer (Japanese Laid-Open Patent Application No. 115964/1989).
However, in the case where these polyether-type copolymers are used, there are existing drawbacks which must be overcome. Specifically, if a tetrahydrofuranethyleneoxide ring-opening copolymer is used, there are problems with water absorption, heat resistance, hot water resistance, and lack of required flexibility because crystalline characteristics remain. If a tetrahydrofuran-propyleneoxide ring-opening copolymer is used, there are problems with respect to heat resistance, ultraviolet ray resistance, and hot water resistance. If a tetrahydrofuran-alkyltetrahydrofuran copolymer is used, the heat resistance, ultraviolet ray resistance, and hot water resistance are good, but the required flexibility is lacking because crystalline characteristics remain. Therefore, these materials do not adequately provide the characteristics required for coatings of optical fibers.
Accordingly, an object of the present invention is to provide, with due consideration to the drawbacks of such conventional compositions, a curable liquid resin composition for use as a covering material for optical fibers, which has a low viscosity at room temperature and exhibits good processability when used to cover optical fibers, has good optical curing characteristics, can accommodate high speed optical fiber production, and, in the cured state, shows excellent flexibility, possesses superior resistance to heat, ultraviolet light, and oil, produces only small amounts of hydrogen gas, and exhibits suitable adherence to optical fiber.